The present invention is concerned with using expression of the CD44 gene or part of the CD44 gene to investigate neoplasia. Such investigation includes taking a tissue, body fluid or other sample from a patient to perform diagnosis, to give a prognosis or to evaluate therapy that is already being carried out. In particular, the invention provides a simple method for carrying out routine screening for neoplasia using body fluid samples or other samples which can be obtained non-invasively.
The usual way to diagnose a tumour at present is by looking at cells or thin slices of tissue down a microscope, a method which is often very effective but has some important limitations. With a small sample, diagnosis can be very difficult and often a large number of cells will not be available, or it is not desirable or possible to obtain a large sample from the patient. In as many as 50% of cases a reliable diagnosis cannot be given; it may be that there is no positive evidence of carcinoma but also no certainty that the patient is actually free from carcinoma. More invasive investigation is then required to establish a diagnosis.
Judgment of prognosis also relies on the appearance of cells when viewed under a microscope. Generally, the more bizarre-looking the cells in a primary tumour, the more likely they are to metastasise later on but the correlation is by no means absolute. It would clearly be an advantage to be able to predict more accurately whether or not metastasis is likely to occur in order to judge what will be the most effective treatment.
The human CD44 gene codes for a family of variably glycosylated cell surface proteins of different sizes, the numerous functions of which are not yet fully established, but which share epitopes recognised by the CD44 monoclonal antibody (mAb). It is known to consist of a standard portion which is expressed in haemopoietic cells and many other cell types and into which the products of additional exons may be spliced in various combinations to produce different proteins. This is a well recognised mechanism in eukaryotes for producing several often functionally unrelated proteins from the same gene, and is known as alternative splicing.
Two common CD44 isoforms have so far been purified and characterised (Stamenkovic et al. 1989), namely i) a 90 kD form consisting of a central 37 kD core which is heavily glycosylated and ii) a 180 kD form which has 135 extra amino acids inserted into the proximal extra-membrane domain and is even more heavily glycosylated. Immuno-cytochemical and immuno-precipitation studies have shown that both are widely distributed in many different cells and tissues. The former is known as the haemopoietic or standard form which is present on circulating leukocytes, bone marrow cells and numerous other cell types. The other, known as the epithelial variant, is detectable on several epithelial cell types. Both are believed to function as receptors mediating homotypic and heterotypic adhesive interactions, attaching cells to each other or to adjacent extracellular scaffolding.
Some time ago, some of the CD44 epitopes recognised by the mAb Hermes-3 were identified as constituting the peripheral lymph node receptor enabling circulating lymphocytes to recognise and traffic through peripheral lymph nodes. Further mAbs to this antigen later became available and Stamenkovic et al. (1989) used one of them to clone a cDNA sequence coding for the standard form of the molecule from an expression library in COS cells. They additionally found, by Northern blotting, that this gene was expressed not only by lymphoid cells, but also by a variety of carcinoma cell lines and a representative sample of solid carcinomas, amongst which two colonic carcinomas appeared to express more than normal colonic epithelium.
Birch and colleagues (1991) reported that melanoma cell clones which strongly expressed the 80-90 kD form of the CD44 antigen, recognised by the Hermes-3 antibody, were substantially more metastatic in nude mice than clones which expressed it weakly. Sy et al. (1991) described a moderate increase in metastatic capability of human lymphoma cells in nude mice, after the cells were transfected with the standard CD44 gene, but not after transfection with a construct coding for the epithelial variant. Gunthert et al. (1991) obtained results indicating that a variant form of the lymphocyte homing receptor, recognised by a new antibody raised to the rat CD44 antigen, is required for metastatic behaviour of rat pancreatic adenocarcinoma cells. Using this antibody they cloned a cDNA sequence corresponding to the variant form of CD44 and found that it contained previously unidentified exons. Transfection of a non-metastatic clone from the same cell line with a construct designed to over-express this cDNA sequence unique to the metastatic counterpart, appeared to induce metastatic behaviour (Gunthert et al, 1991).
In view of these findings it became of interest to know whether other cultured metastatic and non-metastatic human tumour cell lines, of various histogenetic origins, expressed CD44 produces differentially. The expression of genes in cells or tissues can be studied most efficiently and sensitively by making cDNA from cellular messenger RNA and amplifying regions of interest with the polymerase chain reaction, using specific oligonucleotide primers chosen to anneal preferentially to portions of the cDNA corresponding to the gene products. However, subsequent work by Hofmann et al. (1991) and the present applicants using this approach provided results which showed that CD44 expression did not regularly and reliably correlate with the metastatic capability or even tumour forming ability of these cultured cell lines in nude mice. At about this time, three separate groups (Hofmann et al, 1991, Stamenkovic et al, 1991 and Jackson et al, 1992) published sequence data on further splice variants they had found being expressed by this gene in various human cell lines.